This invention is concerned with accurately weighing heated materials, and is especially applicable to a pyrolysis furnace and to the measurement of weight loss in such a furnace.
Certain industrial processes require accurate measurement of the weight or mass of a material before it is in a state of thermal equilibrium. In some cases, it is necessary to achieve accuracy on the order of tenths of a gram with samples larger than 3 kg. For example, the construction industry requires the measurement of asphalt content for quality control purposes. Asphalt is a mixture of asphalt binder and aggregate and is used heavily in the construction of roads. The mechanical properties of this mixture depend on many parameters, such as the asphalt binder content by weight and the gradation of the aggregate. In order to measure the quality of these materials, the contractor needs a process to separate the binder from the aggregate.
In the past, there have been several accepted methods to obtain this information. Two such methods involved chemical solvents and nuclear isotopes. The nuclear asphalt content gauge can be used to accurately measure the binder content of asphalt in just a few minutes. Although this method is fast, the drawback is that gradation analysis cannot be obtained. Chemical solvents can give both asphalt content and gradation analysis. However this method is laborious, time consuming, and the waste solvent poses environmental problems.
In recent years, a method of igniting asphalt in order to measure the weight loss due to combustion has become accepted. Although this method is relatively slow as compared to the nuclear techniques, gradation analysis can be obtained as soon as the ash has cooled. With the advent of new technologies in the construction industry, the standards have also become more stringent. Variations in weight loss measurements from lab to production site to construction site, and even furnace manufacturer, must be minimized.
In the conventional industry process, a sample is weighed to the nearest tenth of a gram using an external scale and placed in a basket assembly. The assembly is then placed in a preheated furnace, which is outfitted with an internal scale assembly or load cell. The door is secured, and the weighing process begins. During the first few moments, a tare or beginning weight is measured. During the next few minutes, the asphalt binder begins to burn and the furnace automatically calculates a weight loss relative to the initial weight and calculates the real time asphalt binder content. The entire process may last from 20 to 60 minutes depending on the initial sample weight and design of the furnace.
Since the asphalt is usually mixed at a temperature of about 150xc2x0 C., and the furnaces are usually preheated to temperatures near 538xc2x0 C., thermal instabilities exist that make the process of obtaining an accurate initial weight of the asphalt a very challenging endeavor. Typically, the errors incurred are on the order of a few grams, and decrease as the sample temperature approaches the temperature of the furnace. The largest error in the weight loss determined using this method is due to an erroneous tare weight obtained during the first few minutes. Generally, the internal scale in the furnace reports a higher basket assembly weight during the first few minutes in the furnace than one would obtain from an external weighing. This error is the direct result of the temperature differential between the furnace and the sample and basket assembly. Furthermore, the last few minutes in the furnace atmosphere are measured as lighter in weight by the internal scale than one would expect externally. Compared to external scale measurements at ambient temperature, the furnace internal scale overestimates the actual weight loss of the sample.
There have been several attempts to clarify the physics of this effect. In one patent, U.S. Pat. No. 5,279,971 to Schneider, the initial error in tare weight is reported as due to moisture absorbed in the asphalt. However, an asphalt plant mixes these constituents at 150xc2x0 C. and moisture accounts for a small percent by weight, if any. Even where the sample is dried overnight and all moisture is removed, the same errors occur. The Schneider patent reports that samples should be preheated to 300xc2x0 C. before placing them in the 550xc2x0 C. furnace. The Schneider patent states that this reduces the xe2x80x9cmoisturexe2x80x9d error. Actually, the error in tare weight was reduced only because the temperature differential between the sample and furnace was 200xc2x0 C. as opposed to 400xc2x0 C.-500xc2x0 C. with a typical sample removed from the production line.
The temperature error caused by placing a relatively cool sample into an extremely hot oven results in a complicated model involving several external factors, such as air density, air flow, and bombardment of the sample and pan assembly by high energy gas molecules. Furthermore, these factors affect the measurement in different ways according to the properties of the sample, such as mass, thermal capacity, thermal conductance, voids or density, and specific gravity. Thus, there are many different combinations of these variables that perturb the initial measurement. There remains a need in the art for a method of accurately weighing samples in a heated furnace that takes into account the complex effects of thermal instability present during the initial weighing process.
The present invention provides a method and apparatus for accurately weighing samples in a heated furnace. More particularly, the present invention provides a method and apparatus in which the weight loss of a sample may be accurately determined as the sample is heated in a furnace. In one specific embodiment, the sample is an asphalt binder/aggregate paving mix and the method and apparatus are utilized to accurately measure the asphalt binder content of the paving mix by determining the weight loss resulting from pyrolysis of the asphalt binder. Using the present invention, weight loss values calculated using the internal scales of a furnace are within about 0.05% of the weight loss values calculated with an external scale.
According to the invention, a correction factor is generated which corrects for errors in the measurement of the tare weight of the sample due to external influences and variables such as those noted above. The invention may additionally correct for errors in the end point weight, also due to external influences. A method in accordance with the broad aspects of the invention, includes the steps of placing a sample in a heated furnace, heating the sample while measurements of sample weight are made, determining a rate function from the sample measurements, producing a weight loss correction factor using the rate function, and using the weight loss correction factor to obtain a corrected weight loss for the sample.
In another aspect, the method includes the steps of placing a combustible sample in a heated furnace, heating the sample while measurements of the weight of the sample are made, determining a weight loss rate function from the sample weight measurements, determining the approximate time at which the onset of sample combustion occurs, producing a weight loss correction factor using the time of combustion onset and the weight loss rate function, and using the weight loss correction factor to obtain a corrected weight loss for the sample.
The weight loss rate function may be suitably determined from the sample weight measurements using regression analysis, such as least squares regression analysis, or other known techniques. During the initial heating of the sample prior to combustion, the weight loss rate may be suitably modeled by a linear function, although other functions could be employed. The time at which the onset of sample combustion occurs can be ascertained in a number of ways. In one embodiment or aspect, combustion onset may be determined by observing the time at which the weight loss rate ceases to be linear, or departs from linear by some threshold amount. In another embodiment or aspect, combustion onset may be determined by monitoring the rate of change in sample temperature or combustion chamber temperature and determining therefrom the projected time at which the sample will reach a known combustion temperature for the particular sample or some other selected temperature. Still another approach involves monitoring the rate of change in sample temperature or combustion chamber temperature and determining the time at which the temperature change rate ceases to be linear, or departs from linear by some threshold amount. Instead of relying upon combustion onset time, it is possible to use other values, such as combustion onset time less 10% or even a fixed time interval. The appropriate method of determining the combustion onset time or other value may depend, in part, on the design of the furnace.
The present invention also provides an apparatus for determining weight loss of a sample, comprising a furnace, a scale mounted within the furnace for measuring sample weight, a data store operatively connected to said scale for storing sample weight measurements, and a weight loss correction factor generator for generating a weight loss correction factor using the sample weight measurements in the data store. The apparatus may also include means for generating a corrected weight loss measurement using a final sample weight measurement from the data store and the weight loss correction factor.
Preferably, the weight loss correction factor generator comprises means for determining a weight loss rate function from the sample weight measurements in the data store, means for determining the approximate time of combustion onset, and means for generating a weight loss correction factor using the time of combustion onset and the weight loss rate function.
Additional features and aspects of the invention will become apparent from the detailed description which follows and from the accompanying drawings, which are intended to be illustrative of the invention, but not restrictive as to the scope and breadth of the invention.